The present invention relates to loudspeaker systems, and more particularly to a dipole type speaker system such as for use in surround sound, reverberation and similar applications.
Various forms of loudspeaker systems have been developed, and the types of speakers as well as the technologies involved pertaining to woofers, tweeters, mid-range and other forms of speaker systems are well known. Stereo sound systems using front speakers with or without some form of woofer or subwoofer, along with rear and/or side speakers, have become prevalent particularly for sound systems used to reproduce sound in "home theater" video systems for playing back video motion pictures and similar program material. The typical installation comprises a pair of front speakers positioned to either side of the TV screen, preferably with a center speaker and/or a subwoofer, and along with a pair of right and left side speaker and/or a pair of left and right rear speakers.
An Audio Engineering Society (AES) paper entitled "New Factors in Sound for Cinema and Television" by Tomlinson Holman, presented at the 89th Convention of the Audio Engineering Society, Los Angeles, Calif., Sep. 21-25, 1990, and reprinted in the Journal of the AES, Volume 39, No. 7/8, (preprint #2945) notes that the best directivity pattern for the "surround" loudspeakers is not the conventional forward radiating direct radiator, but rather dipolar radiation with the principal lobes of the dipole pointed, not at the listening area, but at the room surfaces with the null in the radiation pattern pointed at listeners, and that the best surround loudspeaker is physically invisible. FIG. 1a of the drawings illustrates a simple prior art dipole speaker with baffle, and FIG. 2a illustrates its radiation pattern, and FIG. 1b shows a prior art boxed dipole speaker with FIG. 2b illustrating the radiation pattern thereof. On the other hand, FIG. 1c illustrates a prior art bipolar speaker designed to be omni-directional at low frequencies, and FIG. 2c illustrates the radiation pattern thereof. It will be noted from FIG. 2b that the null is capable of being directed toward the listener (for example, positioned physically where the 270.degree. marking is on the drawing). In this regard, FIG. 2d illustrates a listener 10, along with right 11 and left 12 radiation patterns from dipole speakers of the type shown in FIG. 1a or 1b with the nulls of the two speakers on each side pointed at the listener 10.
Unfortunately, available embodiments of speaker systems meeting the foregoing criteria generally involve mounting a relatively large (e.g. 9 1/2.times.12.times.12 inches, 16 1/2.times.9 1/2.times.7 3/4 inches, etc.) speaker on sides of the room adjacent to the listening area and with five feet being the preferred shortest distance from each side speaker to the listener. Such large size speakers is in some respects essentially mandated by Dolby Laboratories requirements that the surround speaker be capable of reproducing one hundred Hertz and by the suggestion in the above AES paper that dipole radiation is best.
Furthermore, as is well know, bass frequencies reproduced by dipole radiators begin to cancel at frequencies dependent on the size of the baffle on which they are mounted. It has been pointed out in an AES paper entitled "Dipole Radiator Systems" by R. J. Newman presented at the 61st AES Convention, Nov. 3-6, 1978, (AES Preprint No. 1395 (M-1)), as well as other sources, that the baffle size requirements may be reduced if the dipole is mounted against a plane surface. However, this still requires an average baffle dimension of four feet for a pure dipole mounted on a wall for a response down 3 dB at 100 Hz. It is understood that the procedure used by at least one licensee of the Lucasfilm THX Home System is to eliminate the bass response from one of the two bass drivers used to create the dipole radiation pattern by placing a high pass filter in series with only one of the drivers, rolling off its bass response, and thereby eliminating one of the sources used to create the cancellation.